Steelman
Ignoring a common cause
Also called third-cause fallacy, confounding, neglecting a common cause.
Missing that a third factor causes both things.
What it is
Concluding that A causes B when both are actually effects of some third factor C.
The pattern
A and B rise and fall together. Therefore A causes B. (Actually, C causes both.)
An example
“Ice-cream sales and drownings rise and fall together, so ice cream causes drowning.”
Hot weather drives both: more people buy ice cream and more people go swimming.
Why it fails
If C drives both A and B, they’ll move together even though neither affects the other. Changing A would do nothing to B.
When it’s not a fallacy
Most lists skip this part. Calling a fair point a fallacy is a mistake too.
It can be fair when
- Plausible common causes have been measured and controlled for, or a randomized experiment has removed them.
Try one
A question from the app. The wrong answers are its usual look-alikes.
School board meeting. Parent: “Students who took the Bible-as-literature elective scored higher on the AP English exam than students who didn’t.” Teacher: “It’s an extra elective. Most of the kids who sign up are already in honors English and Latin.” Parent: “And they came out ahead. Those stories run through half of English literature. Offer the class in every high school and the scores will follow.”
What’s the flaw?
The teacher pointed to what’s behind both: strong, eager students. They’re the ones who pick an extra literature class, and they’d score well on the exam anyway, so the gap doesn’t show the class raised anyone’s score.
A fair reply
“Compare honors kids who took it with honors kids who didn’t, and if the gap holds, I’m with you.”
Often confused with
- Correlation is not causation
Ignoring a common cause is one specific way the correlation-to-causation leap goes wrong.
- Reverse causation
Reverse causation flips the arrow. A common cause means there’s no arrow between A and B at all.
Where the name comes from
“Common” here means shared, as in common ground: one cause behind both things. “Third-cause fallacy” counts it out: you watched A and B and missed C. “Confounding” comes from the Latin for “to mix together”: a hidden factor mixes its effect in with the one you’re measuring.
Sources
- Fallacies, Bradley Dowden, Internet Encyclopedia of Philosophy
In the app
Practice spotting it on arguments that sound real.
Steelman has more examples of ignoring a common cause, how to answer it in a real conversation, and practice rounds that mix it with its look-alikes, so you learn the difference, not just the name. Coming soon to iPhone and Android.
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